This invention relates to an improved process for making continuous high strength small diameter carbon fibers, and to the fibers made by the process.
Carbon fibers made from mesophase pitch offer desirable properties such as high strength and high modulus. U.S. Pat. No. 4,915,926 discloses process conditions leading to a balance of high strength and high modulus in carbon fibers, but while this patent mentions fibers as small as 5 micrometers in diameter, (see column 5, line 51) the conditions taught therein are not specifically optimized for producing small diameter fibers. U.S. Pat. No. 4,913,889 reports production of carbon fibers having very good strengths and modulus parameters, but because the process employed requires the use of spinnerets with flared outlets, it appears improbable that small diameter carbon fibers could be produced. In spite of the teaching in the art, small diameter continuous pitch based carbon fibers have not been readily commercially available.
Small diameter carbon fibers are desired because they should follow the general relationship of increasing strength with decreasing diameter. However, any process for such small diameter fibers must overcome the problem of the formation of strength reducing axial cracks in pitch based carbon fibers which is well recognized in the art. Small diameter fibers are also sought since the manufacture of carbon fibers involves heat treatments, smaller diameter fibers can be heat treated more efficiently than larger fibers. This invention provides novel continuous pitch based small diameter carbon fibers and a process especially tailored for producing small diameter high tenacity carbon fibers.